The present invention relates to the field of non-cryogenic separation of gases, such as air, into components.
It has been known to use a polymeric membrane to separate air into components. Various polymers have the property that they allow different gases to flow through, or permeate, the membrane, at different rates. A polymer used in air separation, for example, will pass oxygen and nitrogen at different rates. The gas that preferentially flows through the membrane wall is called the “permeate” gas, and the gas that tends not to flow through the membrane is called the “non-permeate” or “retentate” gas. The selectivity of the membrane is a measure of the degree to which the membrane allows one component, but not the other, to pass through.
A membrane-based gas separation system has the inherent advantage that the system does not require the transportation, storage, and handling of cryogenic liquids. Also, a membrane system requires relatively little energy. The membrane itself has no moving parts; the only moving part in the overall membrane system is usually the compressor which provides the gas to be fed to the membrane at elevated pressure.
A gas separation membrane unit is typically provided in the form of a module containing a large number of small, hollow fibers made of the selected polymeric membrane material. The module is generally cylindrical, and terminates in a pair of tubesheets which anchor the hollow fibers. The tubesheets are impervious to gas. The fibers are mounted so as to extend through the tubesheets, so that gas flowing through the interior of the fibers (known in the art as the bore side) can effectively bypass the tubesheets. But gas flowing in the region external to the fibers (known as the shell side) cannot pass through the tubesheets.
In operation, a gas is introduced into a membrane module, the gas being directed to flow through the bore side of the fibers. One component of the gas permeates through the fiber walls, and emerges on the shell side of the fibers, while the other, non-permeate, component tends to flow straight through the bores of the fibers. The non-permeate component comprises a product stream that emerges from the bore sides of the fibers at the outlet end of the module.
The effectiveness of a membrane in gas separation depends not only on the inherent selectivity of the membrane, but also on its capability of handling a sufficiently large product flow. Gas permeates through the membrane due to the pressure differential between one side of the membrane and the other. Thus, to maintain the pressure differential, it is advantageous to remove the permeate gas from the vicinity of the fibers, after such gas has emerged on the shell side. Removal of the permeate gas maximizes the partial pressure difference across the membrane, with respect to the permeate gas, along the length of the module, thus improving both the productivity and recovery of the module. In the membrane module of the present invention, the permeate gas is made to flow out of the module in a direction opposite to that of the basic feed stream.
The removal of the permeate gas is typically accomplished with a “sweep” stream, i.e. a stream of gas which carries the permeate gas out of the module. The sweep gas may also dilute the permeate gas, reducing its partial pressure, and further assisting in the removal of permeate gas from the module.
U.S. Pat. Nos. 4,834,779 and 6,755,894, the disclosures of which are incorporated by reference herein, provide examples of the use of sweep streams.
Conventional gas-separation modules are designed to create a simple counter-current flow pattern between the high pressure retentate gas flowing through the bores of the hollow fiber membranes and the low pressure permeate flowing on the outside of the fibers. That is, the permeate which has passed through the membrane is made to flow in the opposite direction of the feed gas flowing through the bores of the fibers. A sweep stream typically aids this counter-current flow.
The following practical considerations may prevent optimum performance of a gas-separation membrane module.
When the module described above is used to separate air into nitrogen-rich and oxygen-rich streams, the nitrogen concentration of the high-pressure gas inside the fiber steadily increases as that gas flows along the length of the fiber, because oxygen is preferentially permeated through the wall of the fiber membrane. The amount of oxygen removed from the high-pressure stream depends on the intrinsic characteristics of the fiber, i.e. its oxygen and nitrogen permeability, as well as on the difference between the partial pressure of oxygen on the pressurized feed side of the membrane and the partial pressure of the oxygen on the low-pressure shell side of the membrane. In general, this partial pressure differential changes along the length of the module.
In a typical module, more oxygen is permeated near the feed end of the module, because, as the pressurized gas passes along the length of the module, the partial pressure of oxygen decreases in the retentate stream. The result is a concentration gradient on the shell side of the module as well. The oxygen concentration is highest on the shell side near the feed end of the module, and lowest near the outlet end.
The counter-current design of conventional modules is intended to minimize this shell side oxygen concentration gradient. Gas permeated near the outlet end of the module, which has lower oxygen content, acts as a sweep for the feed end of the module that is permeating oxygen at a higher concentration. This arrangement tends to maximize the partial pressure differential along the full length of the module, and helps to maintain the flow of oxygen through the membrane throughout the module.
Notwithstanding this counter-current flow pattern, there will still be regions of the shell side in which the oxygen level will exceed 21%, the percentage in ambient air. When operating to give low purity nitrogen (of the order of less than 95% purity), often the entire length of the shell side of the module has an oxygen concentration of greater than 21%. The latter is due to the fact that there is more oxygen remaining in the retentate stream, near the outlet end, so there is more permeation of oxygen at the outlet end. When the purity of the nitrogen increases, however, the shell-side oxygen concentration at the outlet end is reduced, due to the fact that most of the oxygen has permeated through the membrane, and there is less oxygen remaining on both sides of the membrane. In this situation, the sweep gas is useful only near the feed end of the module.
In general, for those regions where the shell-side oxygen concentration is above 21%, a low-pressure air sweep, formed of ambient air, can be added to increase further the partial pressure differential, and thus to increase the permeate flow.
The above considerations show that, to optimize the operation of the module, the position and flow of the sweep stream could be adjusted according to the concentrations of oxygen and nitrogen in the various streams, at various positions along the length of the module. In many cases, however, the advantage that might be obtained by adjusting the sweep stream would not justify the effort.
However, one application which benefits greatly from careful control of the sweep stream is in the field of aviation. Specifically, membrane-based modules can be used to produce oxygen-depleted gas for inerting of the fuel tanks of an aircraft. The oxygen-depleted gas is conveyed into the unoccupied head space of a fuel tank, to reduce or eliminate the risk of explosion.
In an aircraft, not only are all the considerations discussed above applicable, but the aircraft experiences major changes in external pressure, due to changes of altitude, for which the inerting system must compensate. The application wherein the fuel tanks of aircraft are inerted is known in the industry as OBIGGS, i.e. on-board generation of inert gas for fuel tank inerting.
In the above application, the feed pressure and feed to product pressure ratio are fairly low, and the acceptable level of oxygen in the inert gas stream is fairly high, up to about 12%. These factors work to make the net oxygen partial pressure across the membrane highly dependent on the oxygen concentration on the shell side. Also, the module must operate at different pressures and product purities, to insure that the fuel tank oxygen levels remain below 12%, during take-off, climb, cruise, descent, and landing. The module operates with feed air taken from the bleed air system of the aircraft, and the pressure of this air therefore changes with altitude. The shell-side pressure also changes with altitude since the permeate flow exits the module at ambient pressure. The nitrogen purity requirements also change during the course of the flight, with lower purity needed during the climb and descent portions of the flight, and higher purity required while cruising at altitude. Because of these different requirements, a module with air sweep capabilities must be able to change the position of the sweep along the length of the module during the flight.
For times when low purity nitrogen (i.e. gas having about 12% oxygen) is required, the invention allows for the entire length of the module to be air swept. When higher purity nitrogen is desired (i.e. gas having about 2% oxygen), then the invention allows for only the feed end of the module to be swept, since providing an air sweep along the entire length of the module would hinder its performance.